Minute Virus of Mice (MVM) is an autonomous parvovirus, one of a group of small single stranded DNA viruses which exhibit teratogenic and tumor-suppressive activity in vivo. The efficient expression of the viral genome depends on both a cellular function expressed transiently during the S-phase of the cell cycle and on the interaction between a differentiation specific host cell factor and a cis-acting viral element(s). The Aims of this proposal are primarily three-fold. 1) We will complete the fine mapping of the four major overlapping MVM mRNAs, a prerequisite for sorting out which RNA encodes which protein. 2) We will develop cell lines which constitutively express defined areas of the MVM genome yet remain superinfectible to MVM virons and DNA, in order to complement and propagate virus made mutant in known regions by site specific methods so that interactions with other regions of the genome and with possible regulatory cellular elements can be explored. 3) We will examine the nature of the tissue specific block to MVM replication by identifying and analyzing those cis-acting elements of the viral genome responsible for the cell differentiation specific control of MVM gene expression and/or replication. These experiements will begin to clarify how the lytic cycle of MVM is regulated. Furthermore, since the expression of the MVM genome is dependent upon cell cycle and differentiation specific host cell factors, and because of the extreme parasitic nature of MVM, and understanding of how this viral expression is regulated should yield significant insight into the participating cellular factors and the mechanism of their action, and therefore, into developmentally regulated gene expression in general.